1. Field of the Invention
The device which is the object of the present invention relates to apparatus of the type comprising a "rotating magnet", intended for measuring a variable quantity for example representative of the speed of rotation of a rotating shaft or of the ignition of the cylinders of an engine.
2. Discussion of Related Art
A measuring apparatus of this type currently used in particular for measuring the speed and distance travelled by a motor vehicle in fact comprises a magnet whereof the rotation depends on the variable to be measured and which cooperates with a rotor of conducting material and which is free to pivot and in which eddy currents occur, which are produced by the rotation of the magnet. The magnet and the rotor form what will be referred to hereafter in the description as the "movable arrangement". These currents which are themselves subject to the action of the field, determine a driving torque proportional to the speed of rotation of the magnet. This type of apparatus has numerous advantages: robustness, linearity of the graduation, direct drive of the adder for the movement of the magnet, after suitable gearing-down. The difficult point of the arrangement is the transmission of the movement representing the information to the magnet. The most conventional solution is a transmission using a flexible shaft connecting the rotating shaft of which one wishes to measure the speed, to the axis of the magnet of the movable measuring arrangement. This method has drawbacks, in particular difficulties regarding installation and maintenance. In addition, it does not allow the direct use of signals coming from the ignition device. There has thus been a tendency to replace the mechanical transmission using a flexible shaft by an electrical transmission, which on the one hand uses an emitter device which provides a signal of frequency proportional to the amplitude of the variable to be measured and on the other hand, for example, a motor whereof the average speed of rotation is proportional to the frequency of the current which supplies it, which motor sets the magnet in rotation, the latter in turn bringing about the pivoting of the rotor of the measuring apparatus. However, the use of a motor for driving the movable measuring arrangement still has several drawbacks: motors of the synchronized asynchronous type start solely when the voltage supplied by the transmitter is sufficient, i.e. from a minimal speed. Conversely, stepping motors which have a high torque at very low speed experience a reduction of the torque when the speed increases on account of the increase in its impedance.